The coefficient of friction (also known as the Cp value) of a hydraulic brake system, namely between the brake pad and the brake disk, determines the generation of the brake torque and it is proportional to the brake pressure. Since only the brake pressure can be measured, it is important for a obtaining accurate knowledge of the brake torque on the wheels to estimate the coefficient of friction as accurately as possible. The coefficient of friction can be calculated based on measuring the vehicle deceleration and the brake pressure when the vehicle parameters, such as vehicle mass, are known. There is also a method for estimating coefficient of friction for which the knowledge of the vehicle mass is not required and according to which the longitudinal deceleration generated by a friction brake is compared to the one which is generated by means of an E-machine, whose accurate torque is known.
DE 35 02 050 A1 proposes to measure as a parameter of the braking torque the temperature which is obtained in the braking device in the moment when the braking torque is generated and to evaluate it in an evaluation device.
DE 100 11 270 A1 discloses a method and a device for determining a parameter of a wheel brake which represents the relationship between the braking torque and the brake pressure. The determination of the brake parameter occurs individually for each wheel by calculating wheel-specific variables such as wheel slippage, the wheel contact force, the wheel brake pressure and the wheel radius.
DE 10 2011 085 984 A1 discloses a method for a determination of the coefficient of friction in vehicles on individual wheels, wherein the friction torque is determined according to a predefined torque curve at one wheel of the vehicle and in response to the wheel torque. The current coefficient of friction can be determined from the phase shift between the wheel torque and from the rotational speed of the wheel.
DE 10 2014 226 290 teaches a method for adjusting or controlling a brake actuation parameter BR(FZ) which either uses the clamping force F, or the brake pressure PR, or the clamping path X (xs) of a vehicle brake which is provided with at least one braked wheel R within an electro-hydraulic or electro-magnetic vehicle brake, which is fixed to a chassis of the vehicle with a brake holder and which brakes the wheels by means of friction pads according to the extent of the brake actuation BR(FZ), wherein the wheel circumferential force F (FU) resulting from the contact of the braked wheel with the read surface is measured and the braking force F (FU) is influenced depending on the braking force as a function of the gradient of the wheel circumferential force F (ΔFU/Δs), and wherein the actual value of the regulation is the gradient of the wheel circumferential force F (ΔFU/Δs) and this actual value is adjusted by adjusting the magnitude of the brake actuation BR(FZ) to a predetermined target value TVR(OP(ΔFU/Δs), or to a target value TUR(OP(ΔFU/Δs)+/−R(ΔFU/Δs).
From DE 10 2010 043 320 A1 are known devices and a method for determining a measured variable for a frictional force acting on a disk brake of a motor vehicle. A sensor device arranged on the disk brake measures the displacement of the brake holder that is caused by the effect of the braking force in a direction perpendicular to the axial direction of the brake disk.
One measure for the effective braking torque is determined based on the deformation resulting from the frictional force caused in a direction that is tangential to the surface of the brake disk. DE 10 2016 220 415 proposes a method for estimating the coefficient of friction of a hydraulic brake system of a motor vehicle, wherein the brake torque is adjusted by means of an accurate actuator, the deceleration of the vehicle or of the wheel is measured and the transmission behavior determines the deceleration of the brake torque. Subsequently, a braking torque is set by means of the hydraulic brake system and the deceleration is measured. The actual brake torque can be determined and the coefficient of friction of the brake can be estimated with the inverse of the brake torque delay for a certain transmission conduct.
The present method applies only to brake systems that are provided with a pressure buildup at the level of the main cylinder. In the case of brake systems with a pressure buildup for an individual axle, it is not possible to infer the coefficient of friction of the individual axles. This is because this approach is based on a total vehicle coefficient of friction (based on a fixed ratio of the use of the axes, usually according to an adjusted distribution). However, with a pressure buildup for individual axles, the distribution of the braking torque to the axles can be varied. Different deviations from the standard coefficient of friction may then occur on the brakes of both axles. That is why it is not possible to infer the coefficient of friction on individual axles from the response of the vehicle. In other words, when the total vehicle coefficient of friction is based on a certain distribution, it is not clear which of the axles causes a deviation from a standard coefficient of friction and how large this deviation is.